I see no reason you can't do what you want here. I assume you want to have a separate partition for your /home, at least that's the way I read it. If you do I think that's a good idea. As Chi pointed out, you don't need to duplicate your swap partition. All of your Linux installs will be able to use the same swap partition. As BladeNet pointed out, you can use a common /home partition and a different user name for the Etch install but I don't recommend trying to share /home between distros, especially when you are going to be playing or testing one of them. It also complicates what you have to do to get it all set up to work right.
Before I can really recommend a partition layout I would need to know the size, number, type, use, and placement on your disk of your current partitions. In other words your partition table. I can get some of that from fstab but not all of it.
Let me tell you the problems you need to deal with:
Unfortunately, this is still pretty much a Windows world. Almost all the BIOSs available and on machines still expect to see a primary partition as the first partition on the first bootable disk. Some BIOSs choke and won't even boot if this isn't the case, even if the file system you are booting is somewhere else. Others get confused and cause other problems. I suspect more people have this problem than realize it. Still others work correctly with no problems in this regard.
As far as partitioning goes there are a couple other challenges. You can only have 4 primary partitions on one physical hard drive. You can only have one extended partition on a physical hard drive. Or, you can have 3 primary partitions and one extended partition. You can't sub-divide primary partitions. You can sub-divide the extended partition, (they are sometimes called logical partitions), but the logical partitions must be contiguous. They can't be scattered all over the drive. As you can see this can create some logistics problems when trying to reorganize an existing set-up.
Also, after you partition and reformat any part of your drive, whether as part of another install or independently with Gparted or another partitioning program, you will need to correct the UUID tags in Mint /etc/fstab and also in Mint /boot/grub/menu.lst. This is another issue we can discuss when the time comes. This post is getting long enough as it is. lol
As far as the grub set up. this won't be a difficult chore. When you install Etch do not put grub on the mbr. Put it in the partition /boot is in, usually the / (root) partition. Then open the Mint /boot/grub/menu.lst and put the chain load lines for the new Etch install in place of the old Windows install. This is what one of my chainloader entries looks like:
title Kbuntu Gutsy 7.10
If it won't boot into Etch, then in Mint open a terminal and type:
This of course assumes you have Etch / (root) on the first partition of the first hard drive.
If you want me to help you with a recommendation on the partition table, post the info. I asked for above and I will be happy to look at it.